This invention relates to an improved structure of a cam shaft for use in an engine.
In a hitherto known engine, it has been common in design to employ a crankshaft and a cam shaft arranged such that the former drives the latter through a helical crank gear. In such engine wherein usually the cam shaft is made of cast iron while journal bearing boxes for supporting opposite ends of the cam shaft are made of aluminium, there involves the necessity of maintaining an axial clearance of a certain size between both ends of the cam shaft and its respective bearings in order to secure smooth rotation of the cam shaft even at extremely low temperatures. However, the problem may arise in that when either one of the valves is caused to move by its associated cam (or when the valve is about to open), the cam shaft tends to be biased in an axial direction either upwardly or downwardly under the influence of an axial component of force acting on the helical crank gear, so that the shaft abuts against the end face of that bearing box toward which it has moved, thereby making striking sounds. When the valve then reverses its motion from opening to closing after passing over the point of maximum cam lift, there occurs an instantaneous change of direction in which the force acts in the rotational direction of the cam shaft, so that the axially directed component of force acting on the helical crank gear reverses its axial direction, biasing the cam shaft now in the opposite direction, thereby causing the cam shaft to abut against the end of the bearing opposite to the preceding one. Concerning this axial movement of the cam shaft occurring repeatedly during the engine operation, the result is that the greater the size of clearance (the extent of axial motion) is selected, the higher the level of striking sound being produced becomes. This objectionable noise continues to take place in a repeated manner as long as the engine operates. Empirically, the striking sound tends to be most intensified when the engine operates at a low speed, thus aggravating the problem of noise in the engine operation. Moreover, it has been usual in practice that shim means is applied between the cam shaft and the end face of either one of the bearings so as to permit adjustment of the clearance therebetween to an optimum value since the clearance obtained in manufacture tends to fluctuate in size due to the possible occurrence of accumulative machining tolerances of all related parts.
In the above prior structure, the problem remains that it requires a number of man-hours for shim adjustment and it does not completely eliminate the above mentioned striking sound because the provision of clearance is essentially necessary.
The invention, therefore, aims to settle those problems mentioned above, by reducing the striking sound without the use of shims.